First of all, yes we run a old version, updating isn't an option.
So, the issue.
Yesterday all commenting started taking awfully long time, and after a while the wp-comments-post.php page turns white.
The strange part is the lack of errors in the logs, we run a 3 tier installation with varnish, nginx/php-fpm and the database tables converted to InnoDB. We do have a project to implement MultiDB but haven't had the time.
Anyone got any suggestions? Besides the obvious ones, like to large database etc, we already got that covered. The site is about 24k blogs and around 30k users..